A technique is known in which a battery capacity (SOC) charged from the terminal voltage at current time is specified by using a map showing the relationship between a battery capacity and a battery terminal voltage, and the battery capacity thus specified is displayed in a display unit (see, for example, Japanese Patent Application Publication No. 2011-91879 A).
However, because the previously stored relationship between the terminal voltage and the battery capacity may vary due to deterioration of the battery, when calculating the state of the battery using the map only, the error in the calculated value becomes large so that, when the battery charge state is displayed on a display unit or the like, there is a problem of giving an uncomfortable feeling to the user.